Many kinds of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of them have already been put into practical use. The most widely employed precursor among them is a photoreceptor in which a photoconductive layer containing as main components photoconductive particles, such as zinc oxide, and a resin binder is provided on a conductive support, and a highly lipophilic toner image is formed on said layer surface through an ordinary electrophotographic process. The toner image-formed surface of the photoreceptor is then treated with an oil-desensitizing solution, often referred to as an etching solution, to selectively render the non-image areas hydrophilic, and thus produce an offset printing plate.
In order to obtain satisfactory prints, it is required that the offset printing plate precursor of the above-described type have various properties, such that the photoreceptor can faithfully reproduce an original on the surface thereof; the photoreceptor surface should have a high affinity for an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic, and, at the same time, should have water resistance; and when used as printing plate, the photoconductive layer having a toner image formed thereon should not come off during printing, and should be well receptive to dampening water so that the non-image areas can retain a hydrophilic property sufficient to be free from stains even after a large number of prints have been reproduced therefrom.
These properties are already known to depend upon the ratio of zinc oxide to resin binder in the photoconductive layer. Specifically, when the ratio of zinc oxide particles to resin binder in the photoconductive layer is decreased, the oil-desensitivity of the photoconductive layer surface is enhanced and the background stain is lessened, whereas the internal cohesive force of the photoconductive layer itself is lowered to result in deterioration of printing impression through insufficiency of the mechanical strength. On the contrary, when the proportion of resin binder is increased, the background stain is increased though the printing impression is heightened. Thus the background stain in particular is a phenomenon relating to the oil-desensitivity of the photoconductive layer surface. The ratio between zinc oxide and a resin binder in the photoconductive layer does not solely influence the oil-desensitivity, but it has become apparent that the oil-densitivity also depends greatly on the kind of the resin binder employed.
Examples of known resins include silicone resins as disclosed in JP-B-34-6670 (The term "JP-B" as used herein means an "examined Japanese patent publication"), styrene-butadiene resins as disclosed in JP-B-35-1950, alkyd resins, maleic acid resins, polyamides as disclosed in JP-B-35-11219, vinyl acetate resins as disclosed in JP-B-41-2425, vinyl acetate copolymers as disclosed in JP-B-41-2426, acryl resins as disclosed in JP-B-35-11216, acrylic acid ester copolymers as disclosed in JP-B-3511219, JP-B-36-8510, JP-B-41-13946 and so on. However, the electrophotographic photoreceptors using those resins have some problems, in that: (1) the photoconductive layer is low in chargeability; (2) the image reproduced thereon is poor in quality (in particular, dot reproducibility and resolving power); (3) their photoreceptivities are low; (4) even when subjected to an oil-desensitizing treatment for producing an offset master, the photoconductive layer surface acquires only insufficient oil-desensitivity, to result in generation of background stains on the prints when offset printing is performed; (5) the photoconductive layer is insufficient in film strength, so that, e.g., separation occurs upon offset printing, and hence a large number of prints cannot be obtained; (6) the image quality is apt to be influenced by the environment at the time of image reproduction (e.g., high temperature and high humidity condition), and so on.
As for the offset master, the background stain resulting from insufficiency in oil-desensitization is a particularly serious problem. For the purpose of solving this problem various resins have been developed, and examined for the aptitude for the binder of zinc oxide and the possibility of enhancing the oil-desensitivity. As resins having an effect on improvement in oil-desensitivity of the photoconductive layer, JP-B-50-31011 discloses the combination of a resin prepared by copolymerizing a (meth)acrylate monomer and another monomer in the presence of fumaric acid, which has a molecular weight of from 1.8.times.10.sup.4 to 1.0.times.10.sup.5 and a glass transition point (Tg) of from 10.degree. C. to 80.degree. C., with a copolymer prepared from a (meth)acrylate monomer and a monomer other than fumaric acid; JP-A-53-54027 (The term "JP-A" as used herein means an "unexamined published patent application") discloses a ternary copolymer comprising a (meth)acrylic acid ester having a substituent which contains a carboxylic acid group apart from the ester linkage by at least 7 atoms; JP-A-54-20735 and JP-A-57-202544 disclose quaternary or quinary copolymers comprising acrylic acid and hydroxyethyl (meth)acrylate; and JP-A-58-68046 discloses a ternary copolymer comprising a (meth)acrylic acid ester having an alkyl group containing 6 to 12 carbon atoms as a substituent and vinyl monomer containing a carboxylic acid group. However, even in the cases of using the above-described resins, which are said to have an effect of enhancing the oil-densitivity, the resulting offset masters are still insufficient in resistance to background stain, printing impression, etc., from a practical point of view.
On the other hand, resins of the type which contain functional groups capable of producing hydrophilic groups through decomposition have been examined for an aptitude for the resin binder. For instance, the resins containing functional groups capable of producing hydroxyl groups by decomposition are disclosed in JP-A-62-195684, JP-A-62-210475 and JP-A-62-210476, and those containing functional groups capable of producing carboxyl groups through decomposition are disclosed in JP-A-62-21269.
These patent specifications describe that since the disclosed resins can produce hydrophilic groups by hydrolysis or hydrogenolysis in an oil-desensitizing solution or dampening water used upon printing, the use of those resins as resin binder for lithographic printing plate precursors can not only evade suffering from various problems (including the aggravation of surface smoothness, the deterioration of electrophotographic characteristics and so on) which are thought to be caused by the strong interaction between hydrophilic groups and the surfaces of photoconductive zinc oxide particles when resins originally containing hydrophilic groups themselves are used as the resin binder, but also produce such an effect that the affinity of the nonimage part (which is rendered hydrophilic by an oil-desensitizing solution) for water is further strengthened by the aforesaid hydrophilic groups produced by the decomposition in the resins, to make a clear distinction between the lipophilic image part and the hydrophilic nonimage part, and, at the same time, to prevent printing ink from adhering to the nonimage part upon printing, and thereby to enable the printing of a large number of clear prints free from background stains.
Even those resins, however, do not yet succeed in realizing substantial prevention of background stain and satisfactory printing impression. Specifically, it turned out that when such a resin as to contain hydrophilic group-producing functional groups is used in an increased amount with the intention of further improving the affinity of the nonimage area for water, the question arose as to durability of the resulting printing plate, because the hydrophilic groups produced by decomposition come to rendering the nonimage area soluble in water while increasing the affinity for water.
Accordingly, methods of further enhancing the effects of the affinity of the nonimage area for water, and, at the same time, further heightening the durability have been awaited.